Second-order autocorrelation spectra of XUV free-electron laser pulses from the Spring-8 Compact SASE Source (SCSS) have been recorded by time and momentum resolved detection of two-photon single ionization of He at 20.45 eV using a split-mirror delay-stage in combination with high-resolution recoil-ion momentum spectroscopy (COLTRIMS). From the autocorrelation trace we extract a coherence time of 8 ± 2 fs and a mean pulse duration of 28 ± 5 fs, much shorter than estimations based on electron bunch-length measurements. Simulations within the partial coherence model [Opt. Lett. 35, 3441 (2010)] are in agreement with experiment if a pulse-front tilt across the FEL beam diameter is taken into account that leads to a temporal shift of about 6 fs between both pulse replicas.

The emission of highly charged ions from Xe clusters exposed to intense extreme ultraviolet laser pulses (λ ∼ 52 nm) from the free electron laser in Japan was investigated using ion momentum spectroscopy. With increasing average cluster size, we observed multiply charged ions Xe z+ up to z = 3. From kinetic energy distributions, we found that multiply charged ions were generated near the cluster surface. Our results suggest that charges are inhomogeneously redistributed in the cluster to lower the total energy stored in the clusters.

We have investigated the ionization of the Ar atom by 51 nm extreme-ultraviolet light pulses at the free-electron laser facility, SPring-8 Compact SASE Source test accelerator, in Japan. The angle-resolved photoelectron spectra contain lines due to sequential three-photon double ionization with the second ionization step proceeding via the resonantly enhanced two-photon absorption. The relative intensities of the corresponding photoelectron peaks and their angular dependence are explained in the framework of a three-step model of the process.

, M. et al. (2013) Unusual under-threshold ionization of neon clusters studied by ion spectroscopy. Journal of Physics AbstractWe carried out time-of-flight mass spectrometry for neon clusters that were exposed to intense free electron laser pulses with the wavelength of 62 nm, which induce optical transition from the ground state (2s 2 2p 6 ) to an excited state (2s 2 2p 5 nl ) in the Ne atoms. In contrast to Ne + ions produced by two-photon absorption from isolated Ne atoms, the Ne + ion yield from Ne clusters shows a linear dependence on the laser intensity (I). Combining the results of our recent electron spectroscopy experiments, we suggest that the linear behavior with respect to I is due to the nano-plasma that is formed as a result of Exciton-Mott Transition.

Charge and energy transfer in argon-core-neon-shell clusters irradiated by free-electron-laser pulses at 62 nm.Physical Review A. Atomic, Molecular, and Optical Physics, 86(3) The multiple ionization of Ar-core-Ne-shell clusters in intense extreme-ultraviolet laser pulses (λ ∼ 62 nm) from the free-electron laser in Japan was investigated utilizing a momentum imaging technique. The Ar composition dependence of the kinetic energies and the yields of the fragment ions give evidence for charge transfer from the Ar core to the Ne shell. We have extended the uniformly charged sphere model originally applied to pristine clusters [Islam et al., Phys. Rev. A 73, 041201(R) (2006)] to the core-shell heterogeneous clusters to estimate the amounts of charge and energy transfers.